Stents used for stent placement are classified roughly into two groups: tube stents made from polyethylene, plastic, and others, and metallic stents made from metals. There are variety of tube stents, such as those with or without a side hole and a flap, straight, curved, pig-tailed tube stents.
A tube stent is pushed forward by a pusher catheter inside a human body along a guide wire or inner catheter inserted into the human body. Because a conventional tube stent is designed to be simply pushed by a pusher catheter and cannot be pulled back, a stent must be removed when the stent could not be accurately placed where it should be. This problem has led to development of delivery systems which is capable of pulling back a stent, examples of which includes inventions as shown below.